1. Field of the Invention
The present invention relates to a process of pickling oxide (scale) formed on the surface of a steel-based metallic materials such as carbon steels, low-alloy steels containing a small amount of a softening/hardening elements such as chromium, nickel, niobium, or special steels containing a large amount of chromium, nickel, or the like.
2. Description of the Prior Art
In general, a strip of steel-based metallic material such as carbon steels, low-alloy steels, special steels, or the like, has been hitherto produced by way of the following steps. First, the strip of metallic material is subjected to mechanical descaling treatment such as shot blasting or the like, to remove scale formed on the surfaces of the strip which is hot-rolled or is annealed after hot-rolling. Subsequently, the strip is subjected to a chemical descaling treatment, i.e., pickling treatment, to achieve a complete removal of the scale therefrom, and thereafter, is cold-rolled.
To perform a pickling treatment, a specific kind of aqueous solution has been heretofore selected dependent on the kind of steel to be treated. For example, in a case where a strip of austenitic stainless steel containing nickel is pickled, an aqueous solution of a nitric acid-hydrofluoric acid mixture is employed for the steel strip. However, since a pickling ability of the aqueous solution of the nitric acid-hydrofluoric acid mixture is degraded according to an increase in chromium content of the steel, there arises a problem of intergranular corrosion of a special steel, such as lowalloy steel, a ferritic stainless steel or the like, produced without any annealing operation or merely with a simplified annealing operation and exhibiting a Cr-depleted zone along the grain boundary.
In view of the aforementioned problem, a pickling treatment is generally performed for special steels such as low-alloy steels, ferritic stainless steels or the like, by employing an aqueous solution of hydrochloric acid or an aqueous solution of sulfuric acid. However, it has been considered that it is difficult to accomplish a complete pickling treatment by using the same complete pickling aqueous solution for steel-based metallic materials, each having a different composition, within the short operating time that has been required from the viewpoint of production on an industrial basis.
In consideration of the aforementioned problems, there are known many articles, each disclosing a process of effectively pickling a steel-based metallic material. For example, an official gazette of Japanese Unexamined Publication Patent (Kokai) No. 59-83783 discloses a process of pickling a strip of steel sheet by way of two steps: first, dipping the steel sheet in an aqueous solution of sulfuric acid to remove scale therefrom by dissolving it in the aqueous solution, and second, dipping the steel sheet in an aqueous solution of nitric acid to remove a dirty substance (smut) adhesively deposited on the surfaces of the steel sheet, and at the same time, maintaining the surfaces of the same in the passive state. However, this process requires a long time until the scale is completely removed from the steel sheet by successively dipping it in the aqueous solutions. In addition, this process has a problem in that intergranular corrosion occurs especially when a steel-based metallic material having the Cr-depleted zone along the grain boundary, as mentioned above, is dipped in an aqueous solution of nitric acid.
An official gazette of Japanese Unexamined Publication Patent (Kokai) No. 64-288 discloses a process of pickling a steel-based metallic material by dipping it in an aqueous solution of sulfuric acid-nitric acid mixture. Since this process has a pickling ability as large as one to five times compared with a case where an aqueous solution of sulfuric acid is employed for pickling treatment, it has the advantage that scale can be removed from the surfaces of the metallic material within a shorter operating time, and moreover, intergranular corrosion does not occur with a steel-based metallic material having the Cr-depleted zone along the grain boundary. However, when the aqueous solution of sulfuric acid-nitric acid mixture is employed for practical pickling treatment, there arises a problem in that a quantity of metallic ion such as an iron ion, a chromium ion or the like increases as a part of the metallic material, dissolved in the aqueous solution, and therefore, the composition and the nature of the aqueous solution vary, resulting in the pickling ability being substantially degraded.